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The principle of collegiality was a standard feature of republican magistracies at Rome. Arbitrary power was avoided by ensuring that every magistracy should be filled by at least two officials, and in any case by an even number. They were to possess equal and co‐ordinate authority, but subject to mutual control. Thus a decision taken by one consul was legal only if it did not incur the veto (intercessio) of the other. This principle led to alternation in the exercise of power by the consuls each month. Under the Principate emperors might take as a colleague in their tribunician power (see tribuni plebis) their intended successors, who in many cases were co‐emperors.

The name collegium was also applied to the two great priesthoods of the pontifices and the augurs and to the quindecimviri sacris faciundis, who had charge of the Sibylline oracles (see sibyl). The lesser priesthoods were known as sodalitates (see sodales).